You have probably used an ellipsis in your writing at some point, but you may not know it by that name. The punctuation mark consisting of three dots in a row is called an ellipsis, but let's be honest, it's usually referred to as "dot dot dot." Before I get into the actual usage, let's get the spelling sorted out. An ellipsis is the grouping of three dots serving the purpose of one punctuation mark. The plural is ellipses (with an "e").
The first point I think we should cover is the fact that an ellipsis is made up of three dots. I know, I've said that three times now, but my point is that it's three dots—not four or five or six. Adding more dots doesn't make it more powerful or more suspenseful. It just looks like you nodded off momentarily with your finger on the period/full stop key. To those rare few who for some strange reason use commas in place of periods when making ellipses, stop doing it. It's just not a thing.
Generally, an ellipsis is used if part of a sentence is omitted and what remains can be grammatically connected without losing meaning. Here's an example: "He was unsure about when to use the ellipsis having used it incorrectly in the past and wanting desperately to use it correctly this time." With part omitted: "He was unsure about when to use the ellipsis ... wanting desperately to use it correctly this time."
An ellipsis is also used if a writer is indicating a break in thought or a drifting off. Here is an example:
"I'm so exhausted. I keep nodding off, and ... I can hardly keep my eyes open."
In academic writing, you'll often see ellipses in summaries. In fiction, you'll see them in dialogue. In fiction, ellipses can be used while leaving sentences incomplete, but that is just for effect—say, a pause, suspense or to show a character slipping into sleep. In academic and journalistic writing, ellipses are used to stitch sentences together, so correct grammar and sentence structure remains.
At the risk of sounding contradictory, there is actually one instance when the ellipsis becomes four dots. If a quotation such as dialogue has omitted material indicated by an ellipsis, and it's also the end of the sentence, then an extra dot is added to indicate an actual period. Technically, it's a period followed by an ellipsis, not a four-dot ellipsis. Some style guides will also insert a space between the period and the ellipsis to indicate the two separate punctuation marks. Here's an example:
"As he typed his message, a sudden wave of tiredness overcame him. With his finger on the period button, he drifted off as a period became an ellipsis and grew into a field of dots on the screen. An incoming message snapped him back into reality with its harsh "ding" sound."
With period + ellipsis:
"As he typed his message, a sudden wave of tiredness overcame him. With his finger on the period button, he drifted off. ... An incoming message snapped him back into reality with its harsh "ding" sound."
Not every style guide will have the same spacing rules, but most will agree on the number of dots in that particular circumstance.
Hopefully this helps with understanding the basic use of this little trio. One last note, though—if you're going to use an ellipsis, only use it once per structure. Don't leave your readers dizzy by using it too much.
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